Moulds for use in metal casting



March 3, 197Q A. DUNLOP 3,498,359

MOULDS FOR USE IN METAL CASTING Filed March 8, 1967 2 Sheets-Sheet 1 In venfar AdamDun/qo A fforneys March 3, 1970 A. DUNLOP 3,498,359

MOULDS FOR USE IN METAL CASTING Filed March 8, 196'? 2 Sheets-Sheet 2 Q LL.

[Ill/en for Adam Dun/op United States Patent 3,498,359 MOULDS FOR USE IN METAL CASTING Adam Dunlop, Rotherham, England, assignor, by mesne assignments, to Imperial Metal Industries (Kynoch) Limited, Birmingham, England, a British company Filed Mar. 8, 1967, Ser. No. 621,680 Claims priority, application Great Britain, Mar. 9, 1966, 10,282/ 66; Mar. 19, 1966, 12,217/ 66 Int. Cl. B22c 3/00 U.S. Cl. 164-14 4 Claims ABSTRACT OF THE DISCLOSURE A process for making a mould for use in casting highly reactive metals such as titanium and zirconium in which the mould is impregnated with pure carbon by heating the mould in an atmosphere containing a hydrocarbon gas or vapor.

The present invention relates to the art of metal founding and is concerned with the construction of moulds for casting the more reactive metals such as titanium and zirconium.

The precision casting process based on the lost Wax process is well suited for the production of complicated and accurate castings. In this method a pattern of the casting to be made is formed in wax, thermoplastic material or other suitable materials. Around the pattern a monolithic mould is built up by the application of a layer of a refractory slurry consisting of a finely divided refractory material in a liquid vehicle, usually a partially hydrolysed ethyl silicate solution, the wet coating is dusted with a dry, usually coarser, refractory and finally hardened by air-drying or exposure to a suitable gas such as ammonia. This coating procedure is repeated until a shell of sufiicient thickness is built up. Following this, the pattern is removed by melting out or by dissolving in a suitable solvent. For steel, nickelbase alloys, cobalt-base alloys and the like the refractories used are usually silica, alumina, aluminium silicate such as molochite and siliminite, zirconia, zirconium silicate and the like, the bonding agent frequently being silica sol. The castings produced from such moulds in these alloys have good surface finish and accuracy. However, when such moulds are used for such reactive metals as titanium and zirconium, interreaction between mould and metal occurs with resulting unsoundness and surface contamination in the castings.

Machined graphite moulds have been used to produce titanium castings which are sound and relatively free from contamination. Such moulds are expensive to produce and are restricted to simple shapes. Solid graphite moulds have a further disadvantage in that they have a severe chilling action on the liquid metal, making the filling of fine detail in the casting difficult.

According to the main feature of this invention mould suitable for highly reactive metals such as titanium and zirconium is made of refractory material impregnated with carbon. The mould is preferably made by any suitable lost wax method.

The carbon impregnation may be performed in various ways, and the following examples are given by way of illustration.

EXAMPLE 1 A ceramic shell lost Wax mould is made using Camshell, a proprietary investment material. A wax replica of the casting required is coated by dipping into the Camshell slurry which consists of a partially hydrolysed ethyl silicate solution made into the consistency of thin cream by the addition of 200' mesh aluminium silitil ice

cate. The wet coating is dusted with 30 mesh refractory powder and then hardened by exposure to ammonia gas or by air drying. This coating procedure is repeated about 7 times to build up a refractory shell mould about Aa" to A in thickness. The wax pattern is then removed by melting. The dewaxed mould is impregnated with carbon in the apparatus illustrated diagrammatically in FIGURE 1. Nitrogen in container 1 is fed via a reducing valve 3 and flow meter 5 to the furnace tube 7 via a two-way cock 9 making it possible for nitrogen alone to be passed through or to be mixed with benzene vapour by bubbling through warm liquid benzene from containers in a water bath 13. In operation, with the hot furnace 15 in position A the mould 17 is placed in the-cold furnace tube 7 which is then purged with nitrogen. With nitrogen continuing to flow, the hot furnace is moved to position B. When the mould is at the required temperature, the two-way cock 9 is adjusted so that the nitrogen is bubbled through warm liquid benzene. The benzene vapour on contacting the hot mould dissociates with the deposition of finely divided carbon on the surface and in the pores of the ceramic mould. With a gas flow of 1 /2 litres per hour, a liquid benzene temperature of 55 C. and a furnace temperature of 825 C., after treatment for 5 hours, about 10% by weight of carbon is impregnated into the mould. From such a mould titanium castings can be made having good surface finish and relatively free from surface contamination.

EXAMPLE 2 A ceramic shell lost wax mould is made as described in Example 1. The dewaxed mould is impregnated with carbon in the apparatus illustrated diagrammatically in FIGURE 2. The gas generator part of the equipment is as shown in FIGURE 1. The impregnation chamber consists of a two piece heat resisting steel assembly comprising a base 19 and a cover 21. A heat resisting steel tube 23 leads into the base of the chamber and then to the centre of a perforated carbon block 25. The ceramic shell mould 27 is placed on the carbon block 25 and the cover 21 fitted over the mould into the base, the junction of the cover and the base being sealed with dry sand. A vent 29 is provided in the cover. The heat resisting steel tube is connected to the gas generator by a flexible pipe 31. Nitrogen in container 1 is fed via the reducing valve 3 and flow meter 5 to the impregnation box via the two-way cock 9 making it possible for nitrogen alone to be passed through or to be mixed with benzene vapour by bubbling through liquid benzene. In operation with the impregnation box outside the furnace 33, the equipment is purged with nitrogen, then the box is introduced into the furnace with nitrogen continuing to flow, the mould and box are brought up to furnace temperature when the two-Way cock is adjusted so that nitrogen is bubbled through the liquid benzene. The benzene vapour on contacting the hot mould dissociates with the deposition of finely divided carbon on the surface and in the pores of the mould. When impregnation is completed, the two-way cock is readjusted so that only nitrogen is flowing, the box is then removed from the furnace and allowed to cool while nitrogen fiows. With a liquid benzene temperature of 53 C. and a furnace temperature of 885 C., after 4 hours treatment about 10% by weight of carbon was impregnated into the mould. From this mould, titanium castings were made having good surface finish and relatively free from surface contamination.

EXAMPLE 3 To show the effect of carbon impregnation by the method already described, 5 ceramic shell moulds hav- 3 ing 0, 2%, and 19% carbon impregnation by weight were made and the titanium castings made from these moulds tested for surface contamination, arising from mould/metal reaction. The surface contamination was measured'by micro hardness testing with the results given in the following table:

Carbon Diamond pyramid, Depth of impregnation micro hardness surface of mould, contamination weight percent Surface Core (ins.)

When no carbon was introduced into the mould a high surface hardness was obtained in the casting with a considerable depth of surface contamination, the mould/metal reaction was so extensive as to produce subcutaneous blow holes below the surface of the casting making the cast part quite unusable. The presence of 2% carbon impregnation prevented subcutaneous defects; thus it would be possible by keeping the carbon impregnation at this level to produce castings having a high surface hardness, as cast, which would be of considerable value for application involving resistance to abrasion, erosion and wear. Thus it will be obvious that by controlling the degree of carbon impregnation in in the mould, it is possible by the procedure already described, to control the surface hardness and depth of surface hardness.

While in the example given, benzene has been cited as a means of providing a source of carbon impregnation, it will be obvious that many other hydrocarbons can be used such as butane, propane and the like. Gaseous hydrocarbons may also be used such as acetylene.

While the above examples make use of ceramic shell moulds made by the lost wax process, other moulding methods may be used and the resulting moulds satisfactorily impregnated with carbon as detailed in the following examples.

EXAMPLE 4 Zircon sand mixed with sufficient sodium silicate to form a mouldable mixture is rammed against a conventional pattern. The compacted mould is hardened by gassing with carbon dioxide gas and then stripped from the pattern and impregnated with carbon by the method described in Example 2.

While zircon sand is cited in this example, it will be perfectly obvious to those skilled in the art that silica sand and the like could also be used.

EXAMPLE 5 Carbon powder coated with wax and mixed with phenolic resin powder is moulded under pressure against a conventional pattern. The compacted mould is stripped from the pattern and impregnated with carbon by the method described in Example 2. During the impregnation treatment the wax and phenolic resin are carbonised producing a completely carbonaceous mould.

EXAMPLE 6 A ceramic shell lost wax mould was made and dewaxed according to any of the Examples 1 to 4. It Was then heated to about 1,000 C. in air to eliminate volatile material. Oncooling the mould was saturated with hydrochloric acid vapour by being suspended over boiling acid. The mould was then filled with furfuryl alcohol which had previously been refluxed in air for 48 hours and contained as a polymerisation catalyst 0.25% by volume of 11 N hydrochloric acid. When absorption of the liquid ceased the excess was poured from the mould and the mould heat-treated, to cause resinification and then carbonisation as follows:

12 hours at room temperature; 12 hours at C. in air then cooled to room temperature; heated in nitrogen from room temperature to 900 C. in a period of 12 hours and then held 2 hours at 900 C. followed by cooling to room temperature still in nitrogen. The amount of carbon in the [mould was about 5% by weight.

We claim:

1. In a method for the manufacture of a mould suitable for use with highly reactive metals the steps of:

(a) making a refractory mould by a lost-wax process.

(b) heating the mould in a furnace.

(c) purging the furnace with an inert gas.

(d) impregnating the hot mould with a vapour formed by passing the inert gas through a liquid hydrocarbon so that the heat of the mould causes dissociation of the vapour with the deposition of finely divided carbon on the surface and in the pores of the mould.

2. Method according to claim 1 in which nitrogen is the inert gas and benzene is the hydrocarbon used.

3. In a method for the manufacture of a mould suitable for use with highly refractive metals, the steps of:

(a) making a refractory mould by a lost wax process;

(b) enclosing the mould in a container of heat resistant material;

(0) purging the container with an inert gas;

((1) placing the container with the mould therein in a furnace;

(e) heating the mould in said furnace while continuing passage of the inert gas through the container;

(f) impregnating the hot mould with a vapor formed by passing the inert gas through a liquid hydrocarbon and thence through said container so that the heat of the mould causes dissociation of the vapor with a deposition of finely divided carbon on the surface and in the pores of the mould.

4. A method according to claim 3 in which nitrogen is the inert gas and benzene is the hydrocarbon used.

References Cited UNITED STATES PATENTS 698,889 4/1902 Atha 16412 1,893,286 1/1933 Iredell 117106 X 2,245,651 9/1940 Craig et al. 117-5.2 2,948,034 8/1960 Schneider 164-41 3,075,847 1/1963 Henry et a1 117--5.2 3,126,597 3/1964 Operhall 16426 3,177,084 4/1965 Amstein 1175.l 3,321,005 5/1967 Lirones 16426 FOREIGN PATENTS 750,196 6/ 1956 Great Britain.

J. SPENCER OVERHOLSER, Primary Examiner JOHN S. BROWN, Assistant Examiner US. Cl. X.R. 

